The rapid growth of mobile telecommunications services has created increasing demand for low-cost, low-power and reduced size and weight equipment. One such equipment is found in the transmitters in base stations and terminals for mobile telephony, as well as transmitters for broadcast and other wireless systems. The transmitters employ power amplifiers to amplify a radio frequency (RF) signal to the antenna. Often, the power amplifier needs to be highly efficient, to not overheat, require excessive cooling or draw less power to increase battery time and decrease the cost of power.
One aspect of amplifier efficiency is related to the current and voltage waveforms used by the power amplifier. More specifically, the amplifier efficiency is related to an amount of simultaneous voltage drop over, and current through, a transistor that is part of the power amplifier. A reduction in the voltage drop and current through the transistor results in a reduction in the used power. The reduction in voltage drop and the current through the transistor are indicated by the waveforms of the current and voltage at the transistor. The optimal waveforms for high (maximum) output power are different from the waveforms at low output powers (relative maximum output), and the traditional circuits used for obtaining those waveforms are mutually exclusive. For a better understanding of the exemplary embodiments, the state of the art of power amplifiers is discussed next.
Traditionally, power amplifiers are categorized into different classes according to their linearity and the waveforms for current and voltage. The two major classes of power amplifiers are linear and non-linear power amplifiers. Classes A, AB, B and C correspond to linear power amplifiers while classes D and F correspond to non-linear power amplifiers. In this regard, FIG. 1A shows the output of a linear power amplifier and FIG. 1B shows the output of a non-linear power amplifier. The axes of FIGS. 1A and B use arbitrary units. Classes A, AB, B, C, D and F are discussed next in more detail. Each class is characterized by circuit properties (which are the same for some classes) and a bias applied to the transistor (which may be different between some classes).
A class A power amplifier is defined as an amplifier that is biased so that the output current flows all the time, as shown in FIG. 2, and the input signal drive level is kept small enough to avoid driving the transistor in the cut-off region. In other words, a conduction angle of the transistor is 360°, meaning that the transistor conducts for the full cycle of the input signal. This property makes the class A power amplifier the most linear of all the amplifier types.
A class B power amplifier is an amplifier in which the conduction angle for the transistor is approximately 180°, as shown in FIG. 3. In this class, the current waveform is a half-wave rectified sinewave and the voltage is confined by a parallel resonant tank circuit to be a sine offset by the supply voltage. Ideal transistors for class B operation have constant transconductance above cut-off and the gate or base biased right at the cut-off. The direct current (DC) supply current is then directly proportional to the RF output current (i.e., to the fundamental harmonic component of the half-wave rectified sine). The sinusoidal RF output voltage is also proportional to the RF output current (for the common linear load resistance). This in turn makes the efficiency of the power amplifier proportional to the output signal's amplitude.
The efficiency of the constant supply voltage class B amplifier is proportional to the output voltage, up to a maximum of 78.5% for ideal devices in a lossless circuit. The average efficiency for a class B amplifier, outputting a signal whose average signal level is low compared to the maximum (peak) level, i.e., that has a high peak-to-average ratio, is therefore low compared to the efficiency at a maximum output.
Some transistors (hereinafter called “quasi-linear”) do not have constant transconductance above cut-off, but can be seen as having a first substantially linear part followed by a substantially constant part (above which it usually drops off again). In this case, the best linearity is obtained by so called class AB bias, shown in FIG. 4. A constant (DC) bias voltage applied to the gate or base is chosen as the point on the transconductance slope around which the transconductance is maximally (odd) symmetric. In the simplified model shown in FIG. 4, this point is exactly in the middle of the linear slope part of the transconductance curve.
Since the gate bias for class AB is in the middle of the transconductance slope, the transistor draws current even if no RF signal is present. Thus, the efficiency of the power amplifier is therefore lower (especially for low signal amplitudes) for a class AB amplifier than for a class B amplifier described in FIG. 3. The current pulses of the class AB power amplifier are similar to the half-wave rectified sinewave pulses of class B but complemented with “tails” on each side.
When biasing a quasilinear transistor so that the transistor draws no DC current when no RF signal is present, the output response is nonlinear with respect to the input. The efficiency is thus better than for the ideal class B amplifier, but the maximum output is lower (for otherwise equal conditions). The current waveform consists of narrower pulses than for ideal class B or AB power amplifiers, which indicates a greater content of higher harmonics.
For most transistors, for example those with constant transconductance or quasi-linear transconductance, decreasing the bias below the cut-off voltage gives even narrower current pulses, lower gain, better efficiency, and further reduced maximum output power. This mode of operation defines the class C power amplifier. The class C power amplifier is an amplifier in which the conduction angle for the transistor may be less than 180° and the characteristic output of this amplifier is shown in FIG. 5. Alternatively, the class C power amplifier is characterized by having narrow output current pulses at a transistor of the power amplifier.
By increasing the gate bias above class AB, to the point that the current is continuously drawn even at high amplitudes, a class A power amplifier is obtained. This class, as discussed above, has low efficiency for small signals, since the DC supply current drawn is high and largely independent of the signal level. The class A power amplifiers are used, inspite of the low efficiency, because they have a higher gain than other classes, which makes it possible to reach a maximum output with a small drive amplifier. For quasilinear transistors, the maximum RF output current of class A amplifiers may be larger than for class B or AB power amplifiers, for the same limitation in instantaneous drain (collector or, more generally, the active device's output node) current. By correspondingly decreasing the load resistance (as seen by the transistor) the maximum output power can be increased.
The abovementioned classes of operation may be obtained by a same circuit, which is characterized by a RF tank (parallel resonator at the fundamental frequency in parallel with the load). The RF tank is necessary because in coupling a tuned RF power amplifier to its load (the antenna or antenna feed line), the correct load resistance, which will enable the amplifier to deliver its rated power, has to be present to the output of the power amplifier. The RF tank ensures that this condition is fulfilled by shorting the harmonics of the current waveform, thereby forcing the output voltage to be sinusoidal. Other functions of the output circuit are to provide the DC current to the transistor through the RF choke and to block the DC from the supply voltage from reaching the load. Since the circuits for the above classes are identical, the difference between these classes of operation lies in the difference in current waveforms, which is caused by different (DC) gate biases to the transistor.
Using a dynamic grid (gate, base) biasing technique to achieve a varying class C through class A operation in a power amplifier, to obtain modulation as well as high average efficiency and high output power, is described in F. E. Terman and F. A. Everest, “Dynamic-shift grid-bias modulation”, Radio, no. 211, pp. 22-29,80-, July, 1936. Improvements to this scheme, to further increase the efficiency and linearity for multicarrier signals, are described in Swedish Patent, SE 200201908 by R. Hellberg.
Non-linear amplifiers include class D and class F amplifiers and these amplifiers will be discussed later in more detail. An increase in the maximum power and efficiency relative to the linear amplifiers may be achieved by the class F and inverse F type amplifiers. The class F and inverse F amplifiers are characterized by a square-shaped output voltage waveform at a transistor of the power amplifier. The effect of class F/inverse F is to obtain a more square-shaped voltage or current waveform as described in V. J. Tyler, “A new high efficiency high power amplifier”, Marconi Review, vol. 21, no. 130, pp 96-109, Third Quarter 1958 and F. H. Raab, “Maximum efficiency and output of class-F power amplifiers”, IEEE trans. MTT, vol. 49, no. 6, June 2001. Using a push-pull arrangement together with switching transistors to obtain the same results as in the class F amplifier is a class D amplifier, i.e., the waveforms are similar but not necessarily the amplifiers. A voltage-switching class D power amplifier corresponds to a class F power amplifier and a current-switching class D power amplifier corresponds to an inverse class F power amplifier.
If the transistor output of the power amplifier “sees” a high impedance (ideally an open circuit) at odd harmonic frequencies instead of short circuits at all harmonics, then a class F power amplifier is present. The high impedance for the odd harmonic frequencies may be obtained by inserting parallel resonators (“harmonic block”) in series with the previously described harmonically shorted load (tank and load). Thus, the class F power amplifier may have a high impedance at odd harmonics (often only the third). Instead, if some even harmonics (often only the second) “see” a high impedance, then an inverse class F power amplifier is present. If more harmonics are to have the high impedance, more parallel resonators are inserted in series, while keeping the remaining harmonics shorted.
The thus flattened waveform has a higher fundamental (RF) amplitude than the peak voltage or current would otherwise allow. This is due to the odd (voltage or current) harmonics that are in antiphase with the fundamental (voltage or current) at its peak, thus suppressing the peak without reducing the fundamental (i.e., flattening the wave towards a more square-shaped form). The high harmonic impedance caused by the parallel resonators imply that even a small harmonic current due to the current pulse shape causes a large voltage. This process is self-regulating in the maximum amplitude region due to the instantaneous current reduction due to voltage saturation (“I-V curve behaviour”).
The class BD amplifier uses linear class B operation at low and medium amplitudes, but increases the maximum output power by using current-switching class D. The class BD amplifier is described in F. H. Raab, “The class BD high-efficiency RF power amplifier”, IEEE Journal of Solid-State Circuits, vol. SC-12, no. 3, June 1977. The class BD power amplifier does not use class C or dynamic gate biasing.
The above discussed classes of power amplifiers have their problems, i.e., either low efficiency or low output power. In other words, no traditional method or power amplifier combines the high efficiency of class C amplifier (through dynamic gate bias) at medium output amplitudes and the high maximum output power of class F or inverse F amplifier. What prevents a combination of the class C amplifier with a class F amplifier is the negative interaction between the narrow current pulses of class C (and with quasi-linear transistors even class B) operation with the high harmonic impedances required for class F or inverse F operation. This problem manifests itself as a large harmonic voltage (by one or several harmonics) in phase with the fundamental voltage, saturating the transistor already at very low output amplitudes. Thus, the efficiency of the power amplifier is, on average, not higher than linear class B efficiency, and the advantage of class C operation cannot be obtained.
Another problem associated with existing power amplifiers that try to operate a class F or inverse F amplifier in class C is that the high harmonic voltages combine to give high and low peak voltages, which are not self-regulated by the saturation (I-V) behaviour of the transistor. Depending on the transistor type, these high and low peaks may cause severe efficiency degradation, failure to reach the desired output power, or catastrophic breakdown. The current and voltage (arbitrary scale) of an RF cycle for a medium output amplitude is shown in FIG. 6.
FIG. 7 shows the maximum and minimum drain voltages versus amplitude for the power amplifier. This figure shows an amplitude sweep of a class F amplifier with dynamic class C excitation. There are large voltage overshoots (r) and undershoots (b) at all amplitudes except the extreme low and high.
Accordingly, it would be desirable to provide devices and methods for communication equipment that avoids the afore-described problems and drawbacks.